The Culprit In Rising Western U.S. Smog Levels: AsiaU.S. emissions of smog-forming pollutants have dropped, but smog levels in the western U.S. have increased each year. Now, researchers say, they've found out why — it's wafting from across the Pacific Ocean.

U.S. emissions of smog-forming pollutants have dropped, but smog levels in the western U.S. have increased each year. Now, researchers say, they've found out why — it's wafting from across the Pacific Ocean.

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Air pollution from sources within the U.S. has declined over the past few decades. But one pollutant to ozone has been increasing in parts of the West. Now, researchers say they've found out why. As NPR's Rae Ellen Bichell reports, a lot of it is wafting over from Asia.

RAE ELLEN BICHELL, BYLINE: Ozone is good when it's way up high in the atmosphere, where it absorbs ultraviolet radiation. It's not so good down on the ground. There, says Arlene Fiore, it can cause breathing problems.

ARLENE FIORE: Ozone also damages plants as well. And so exposure to ozone can reduce the yields, for instance, of agricultural crops and things like that.

BICHELL: Fiore is an atmospheric chemist with Columbia University and the Lamont-Doherty Earth Observatory. Over the last few decades, she says, peak ozone levels have gone down in many parts of the country, in line with policies created to reduce pollution.

FIORE: And so I think there's a clear success story here of the, you know, the major efforts that have gone into place to improve air quality over the U.S.

BICHELL: But in parts of the Western U.S., ozone levels have been rising. After looking at data over the last 35 years at sites across the country, Fiore and her colleagues think they've figured out why.

FIORE: Some of the gains made under these emission control programs domestically are being offset by rising emissions elsewhere in the globe.

BICHELL: And by elsewhere, she really means Asia, particularly countries like India and China. The U.S. emits about half as much ozone-producing pollution as it did a couple of decades ago, but Asian emissions have tripled. The impact here in the U.S. is felt especially at this time of year. During the spring, storms can lift ozone from Asia and carry it across the Pacific Ocean. The impact eclipses that of domestic ozone-producing sources.

FIORE: And so this idea that, you know, the U.S. has full control over the air that we're breathing isn't entirely true because it's a global atmosphere. And it's interconnected. And it's part of our climate system.

BICHELL: The findings were published this week in the journal "Atmospheric Chemistry And Physics." Rae Ellen Bichell, NPR News.

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